There is little difference in the temperature in Jamaica throughout the year with average winter temperatures of 25°C and summer averages of 28°C. Mornings and evenings can be a bit cooler in winter but this is generally pleasant rather than cold and long sleeves will suffice. Rain is frequent in the north-east but showers are brief outside the rainy seasons. May and August - October are the rainy months, although even then it doesn't rain every day.
The most pleasant time to visit Jamaica is winter, when the Jamaican heat is not too fierce. This corresponds to the peak tourist season from December to mid-April as tourists flock from colder climates for some winter sun. Prices are at their highest during this period although booking early can still get you a good deal. The heat of the summer in June and July can be a great time to visit with fewer crowds and cheaper prices, particularly if exploring the mountains and interior appeals more than sunbathing, as the altitude keeps the temperatures down.
Many festivals in Jamaica are movable feasts with organisers prone to changing the date or location if it looks too hot or too rainy, but there are plenty to aim for if you want to party with the locals. For a traditional festival the Accompong Maroon Festival is held on 6th January, or for a party that sweeps the whole island you can't go wrong with Carnival, held the week before Easter. Music festivals are popular too with Reggae Sumfest in Montego Bay in August and the Rebel Salute Music Festival in Kingston in January often has international stars. Sporting events like the Jamaican Cricket Festival or local events like the Breadfruit Festival in Bath add to the variety.